Diagnosis: General locomotive shape while moving is arcate, however peripheral contours change continuously, especially during rapid locomotion. Dimensions variable owing to very rapid changes of shape during active locomotion; during rapid locomotion, 21-42 µm wide, 12-25 µm long. Ectoplasma thin, clear over body surface, forms subpseudopodia and leading margin of body; posterior edge often adheres to slide and stretches briefly as ragged, trailing pseudopodial filaments during locomotion. Endoplasm clear, faintly granular, some light-refractile granules. Many short, clear, populate, rounded subpseudopodia, 3-5 µm long on upper surface; those along the leading edge 3-6 µm long, formed singly or in pairs, occasionally fusing into the clear, narrow leading border during locomotion. Nucleus: single pale with indistinct outline, about 3.5 µm in diameter, in a thin clear perinuclear vesicle, with no distinct nucleolus; usually near the center of body mass. Usually one contractile vacuole. Crystals very small. No cysts observed.